Polymer dispersed liquid crystal (PDLC) films, consisting of liquid crystal microdroplets dispersed in a polymer matrix, are potentially useful for solar energy control and other electro-optic applications, such as sunroofs, solar windows and information displays. Generally, these materials are formed by the incorporation of liquid crystals in a cross-linked epoxy binder, or in a polymer matrix which has been cured using thermal, ultraviolet or electron-beam methods. These films can easily be switched from an off-state which is cloudy, opaque, and light scattering, to an on-state which is essentially transparent. Most often this switching is accomplished by application of a suitable electrical voltage across the thickness of the film. However other methods for accomplishing this change in transparency of the film include the application of heat or stress, or alternatively the application of a magnetic field across the thickness of that portion of the film where transparency is desired.
The usefulness of a particular polymer dispersed liquid crystal film depends on both the magnitude of light scattering by the film and the directions into which the light is scattered. For windows, sunroofs and other devices which are formed from these polymer dispersed liquid crystal films and which are designed to control the transmission of solar radiation, including heat and light, it is necessary to maximize the amount of radiation backscattered from the film so as to minimize the amount of heat and light transmitted through the film during its off or scattering state when no electrical potential is applied.
Optimizing the solar heat load attenuation and light scattering performance of a polymer dispersed liquid crystal film for a specific application requires control over the various film parameters. These parameters include not only the refractive indices of the liquid crystal and polymer matrix materials, but also the concentration and size distribution of the liquid crystal droplets in the film, and further the fraction of the initial liquid crystal displaced into the droplets by polymerization of the polymer precursor. Most of these critical film parameters are established during the cure of the polymer. These parameters depend on the cure conditions which influence the cure kinetics (i.e., temperature and radiation intensity), as well as on the exact chemical composition of the polymer precursor including any additives.
Much work has been done in this field to optimize the light scattering and solar heat load attenuation characteristics of these polymer dispersed liquid crystal films so as to produce films which are capable of reducing solar heat and light transmission for such applications as automotive sunroofs and windows as well as building windows and structures. However, the research has failed to optimize those cure conditions which will result in the desired film qualities. To date, current state of the art polymer dispersed liquid crystal films have been characterized by solar attenuation properties which are moderately successful. Generally, these conventional films reflect (or backscatter) about 15 to 20 percent of the incoming solar radiation, and transmit through the film about 40 to 65 percent of the incoming solar radiation. The remainder of incoming solar radiation is essentially trapped within the film itself. For widespread automotive and commercial use, these conventional polymer dispersed liquid crystal films should have improved solar attenuation properties; in particular the films should reflect more and thereby transmit less of the incoming solar radiation.
Therefore, it is desirable to provide a polymer dispersed liquid crystal film for use in these types of devices which effectively controls the transmission of solar radiation so as to maximize the backscattering and attenuation of the incoming solar radiation, and thereby accordingly minimize the amount of solar radiation (in the form of heat and light) transmitted through the film. It would also be desirable to minimize the cost of such a film so as to be competitive with the cost for producing conventional films. This would probably best be accomplished by limiting or reducing the amount of liquid crystal component within these films, since the liquid crystal component is the most expensive component within these films.